In gaseous flows, the phenomena of compression exists and has a large effect. It allows the number of molecules for a given volume to change with pressure and temperature as well as with composition. Therefore, it is desirable to make natural gas sales transactions either by mass, energy, or at standard pressure and temperature conditions. In the U.S., for example, the standard pressure and temperature of gas is stated as 14.7 psia and 60.degree. F. for many transactions. Delivery calculations state the flow adjusted to correspond to these base conditions even though the actual gas in the transaction is probably at a different pressure or temperature. A piece of equipment designed to accomplish the task of converting a measured volumetric flowrate Q.sub.f to a base volumetric flowrate Q.sub.b at a defined pressure and temperature is referred to as a "volume corrector".
In the traditional method of gas measurement, a volume correction ratio ##EQU1## is determined from the pipeline gas flow temperature, pressure, and composition using the following relation: ##EQU2## where Q.sub.f is the measured volumetric flowrate of the pipeline gas through the pipeline, T.sub.b and P.sub.b are the base condition temperature and pressure (e.g. 14.7 psia and 60.degree. F.), T.sub.f and P.sub.f are the measured flow temperature and pressure of the pipeline gas in the pipeline, Z.sub.b and Z.sub.f are the supercompressibility factors at the base condition and the flow condition, respectively, and Q.sub.b is the base condition volumetric flowrate. Such a calculation is typically carried out in a flow computer.
Using the relation in Eq. (1) to compute base condition volumetric flowrate Q.sub.b requires high accuracy in the measurement of the flow temperature T.sub.f and pressure P.sub.f. This requires that pressure and temperature sensors for monitoring P.sub.f and T.sub.f be calibrated frequently.
The ratio ##EQU3## in Eq. (1) presents even more difficulties. The composition of the gas is normally measured by gas chromatography and the supercompressibilities, Z.sub.b and Z.sub.f, are estimated from either the virial equations of state, or from pre-calculated correlations such as NX-19 or the more recent Gergg Equations. Alternatively, a meter that measures heating value, relative density, %CO.sub.2 and %N.sub.2 can be used to calculate the ratio ##EQU4## This is because the Gergg Equations in their short form allow calculation of the ratio ##EQU5## from these parameters.
Knowledge of the values of the virial coefficients of particular gas compositions is quite limited so calculation of supercompressibility from the virial equation of state is not always possible. The Gergg Equations and NX-19 correlation are mathematical models obtained by mapping known and measured properties. The Gergg Equations, in particular, are very good over a wide range of compositions. Use of the Gergg Equations, however, requires either a chromatograph or a special meter to measure the properties needed to solve the short form Gergg Equations, both of which are expensive.
It is, therefore, difficult to obtain accurate measurement of the supercompressibility ratio ##EQU6## in a cost effective manner.
In U.S. Pat. No. 5,201,581, patented Apr. 13, 1991; U.S. Pat. No. 5,226,728, patented Jul. 13, 1993, and U.S. patent application Ser. No. 08/009,481, filed Jan. 25, 1993; Vander Heyden and Clingman disclose other inventions that can determine a volumetric flow correction ratio. These inventions by Vander Heyden and Clingman alleviate the need to compute supercompressibility when determining a volumetric flow correction ratio for a pipeline gas. These inventions also alleviate the need to measure the absolute temperature, absolute pressure, and composition of the pipeline gas. They can do this by tapping sample gas from a pipeline, maintaining the temperature of the sample gas at substantially the same temperature as the pipeline gas flowing through the pipeline, and measuring the flowrate of the sample gas or its equivalent while the sample gas is being maintained at substantially the same temperature as the pipeline gas. These inventions by Vander Heyden and Clingman operate accurately to determine energy flowrates, volume correction ratios ##EQU7## and adjusted or base condition volumetric flowrates Q.sub.b.
The inventions by Vander Heyden and Clingman referred to above require, however, that the sample gas flowmeter respond to density and density effects in the same manner as the pipeline gas flowmeter. This means that a volumetric flow corrector having a differential pressure sample gas flowmeter (such as a capillary with a differential pressure cell) which is made to be used with a differential pressure pipeline gas flowmeter (such as an orifice meter) cannot be used in another application where the pipeline gas meter is a linear flowmeter (such as a turbine meter). The flexibility of such a volumetric flow corrector is thus limited The same is also true for a volumetric flow corrector having a linear sample gas flowmeter.
The present invention can be used in conjunction with either a differential pressure pipeline gas flowmeter or a linear pipeline gas flowmeter. At the same time, it continues to alleviate the need to compute the supercompressibility of the pipeline gas, and also the need to measure the absolute temperature, the absolute pressure, and composition of the pipeline gas.